Acoustic logging in low velocity formations

Claim 3

The method of claim 1 wherein said gas bubbles are introduced into the liquid zone from a source located below the zone.

Claim 4

The method of claim 1 wherein the gas bubbles are introduced through a single gas injector hole.

Claim 7

The method of claim 4 wherein the gas bubbles are introduced through a gas injector ring, said ring having at least one hollow spoke, a hollow outer edge, and at least one perforation on said outer edge through which gas bubbles may escape into the liquid.

Claim 8

The method of claim 5 wherein two or more perforations are spaced substantially evenly from each other on said outer edge.

Claim 9

The method of claim 5 wherein said injector ring is located directly below the logging device.

Claim 10

The method of claim 5 wherein said injector ring is attached by a hollow tube to a compressed gas source on the ground above the borehole, said hollow tube having a first open end attached to said gas compression source and a second open end attached to said injector ring.

Claim 11

The method of claim 4 wherein a hollow tube is attached to the logging sonde, said tube having a first open end through which gas bubbles are introduced into the liquid zone, and a second open end attached to said compressed gas source.

Claim 12

Apparatus for acoustically logging an earth formation surrounding a borehole which contains a liquid, said apparatus, comprising: a logging sonde; an acoustic source attached to said logging sonde, and capable of generating P-waves having a predetermined frequency at a first location in the borehole so that said P-waves will propagate through a zone in said liquid and then penetrate the formation and be refracted in the formation; means attached to the logging sonde for introducing gas bubbles into the liquid zone so as to lower the P-wave velocity in the liquid zone to a value less than the P-wave velocity in said formation, wherein the average resonant frequency of said bubbles is selected to be greater than said predetermined frequency, and wherein the average diameter of the bubbles is determined by the following equation: F.sub.R =1/πD{3μΥPo/ρ}.sup.1/2 where F R is the average resonant frequency of the bubbles; D is the average diameter of the bubbles; Po is the pressure at the bubbles; ρ is the density of the water gas mixtures; μ is the Polytropic factor; and Υ is the adiabatic constant; and at least one receiver attached to the logging sonde, said rceivers being capable of detecting the arrival of the refracted P-waves.

Claim 13

Apparatus for acoustically logging an earth formation surrounding a borehole which contains a liquid, said apparatus, comprising: a logging sonde; an acoustic source attached to said logging sonde, and capable of generating P-waves having a predetermined frequency at a first location in the borehole so that said P-waves will propagate through a zone in said liquid and then penetrate the formation and be refracted in the formation; means attached to the logging sonde for introducing gas bubbles into the liquid zone so as to lower the P-wave velocity in the liquid zone to a value less than the P-wave velocity in said formation, wherein the average resonant frequency of said bubbles is selected to be greater than said predetermined frequency, and wherein the average diameter of the bubbles is determined by the following equation: F.sub.R =1/πD{3μΥPo/ρ}.sup.1/2 where F R is the average resonant frequency of the bubbles; D is the average diameter of the bubbles; Po is the pressure at the bubbles; ρ is the density of the water gas mixtures; μ is the Polytropic factor; and Υ is the adiabatic constant; and at least one receiver attached to the logging sonde, said rceivers being capable of detecting the arrival of the refracted P-waves.

Claim 14

The apparatus of claim 10 wherein said gas bubbles introducing means is attached so as to be located below the first location when the apparatus is positioned in the borehole.

Claim 15

Apparatus for acoustically logging an earth formation surrounding a borehole having an axis and containing a liquid, said apparatus comprising: a logging sonde; an acoustic source attached to said logging sonde, and capable of generating P-waves having a predetermined frequency at a first location in theborehole so that said P-waves will propagate through a zone in said liquid and then penetrate the formation and be refracted in the formation; a hollow tube attached to the logging sonde and having a first open end and a second open end; a compressed gas source attached to said second open end; a gas injector ring attached to said second open end, said ring having at least one spoke extending from said tube, an outer edge, and a plurality of perforations on said outer edge, said perforations capable of introducing gas bubbles into said liquid zone having an average resonant frequency selected to be greater than said predetermined frequency, so as to lower the P-wave velocity in the liquid zone to a valve less than the P-wave velocity in said formation, wherein the average diameter of the bubbles is determined by the following equation: F.sub.R =1/πD{3μΥPo/ρ}.sup.1/2 where F R is the average resonant frequency of the bubbles; D is the average diameter of the bubbles. Po is the pressure at the bubbles; ρ is the density of the water gas mixture; μ is the Polytropic factor; and Υ is the adiabatic constant; and at least one receiver attached to the logging sonde and spaced vertically along said logging sonde from said acoustic source for detecting the arrival of the refracted P-waves.

Claim 16

Apparatus for acoustically logging an earth formation surrounding a borehole having an axis and containing a liquid, said apparatus comprising: a logging sonde; an acoustic source attached to said logging sonde, and capable of generating P-waves having a predetermined frequency at a first location in theborehole so that said P-waves will propagate through a zone in said liquid and then penetrate the formation and be refracted in the formation; a hollow tube attached to the logging sonde and having a first open end and a second open end; a compressed gas source attached to said second open end; a gas injector ring attached to said second open end, said ring having at least one spoke extending from said tube, an outer edge, and a plurality of perforations on said outer edge, said perforations capable of introducing gas bubbles into said liquid zone having an average resonant frequency selected to be greater than said predetermined frequency, so as to lower the P-wave velocity in the liquid zone to a valve less than the P-wave velocity in said formation, wherein the average diameter of the bubbles is determined by the following equation: F.sub.R =1/πD{3μΥPo/ρ}.sup.1/2 where F R is the average resonant frequency of the bubbles; D is the average diameter of the bubbles. Po is the pressure at the bubbles; ρ is the density of the water gas mixture; μ is the Polytropic factor; and Υ is the adiabatic constant; and at least one receiver attached to the logging sonde and spaced vertically along said logging sonde from said acoustic source for detecting the arrival of the refracted P-waves.

Claim 17

The apparatus of claim 12 wherein two or more perforations are spaced substantially evenly from each other on said outer edge.

Claim 18

The apparatus of claim 12 wherein the mean diameter of the perforations is such that the gas bubbles introduced into the liquid through the perforations have an average resonant frequency greater than said predetermined frequency of the P-waves.

Claim 19

The apparatus of claim 12 wherein said injector ring is located below the first location when the apparatus is positioned in the borehole.

Claim 20

Apparatus for acoustically logging an earth formation surrounding a borehole having an axis and containing a liquid, said apparatus comprising: a logging sonde, an acoustic source attached to said logging sonde and capable of generating P-waves having a predetermined frequency at a first location in the borehole so that said P-wave will propagate through a zone in said liquid and then penetrate the formation and be refracted in the formation; a hollow tube attached to the logging sonde and having a first open end with a single injector hole located at said first open end, said injector hole capable of introducing gas bubbles into said liquid zone having an average resonant frequency selected to be greater than said predetermined frequency, so as to lower the P-wave velocity in the liquid zone to a value less than the P-wave velocity in said formation, wherein the average diameter of the bubbles is determined by the following equation: F.sub.R =1/πD{3μΥPo/ρ}.sup.1/2 where F R is the average resonant frequency of the bubbles; D is the average diameter of the bubbles; Po is the pressure at the bubbles; ρ is the density of the water gas mixture; μ is the Polytropic factor; and Υ is the adiabatic constant; and at least one receiver attached to the logging sonde and spaced vertically along said logging sonde from said acoustic source for detecting the arrival of the refracted P-waves.

Claim 21

Apparatus for acoustically logging an earth formation surrounding a borehole having an axis and containing a liquid, said apparatus comprising: a logging sonde, an acoustic source attached to said logging sonde and capable of generating P-waves having a predetermined frequency at a first location in the borehole so that said P-wave will propagate through a zone in said liquid and then penetrate the formation and be refracted in the formation; a hollow tube attached to the logging sonde and having a first open end with a single injector hole located at said first open end, said injector hole capable of introducing gas bubbles into said liquid zone having an average resonant frequency selected to be greater than said predetermined frequency, so as to lower the P-wave velocity in the liquid zone to a value less than the P-wave velocity in said formation, wherein the average diameter of the bubbles is determined by the following equation: F.sub.R =1/πD{3μΥPo/ρ}.sup.1/2 where F R is the average resonant frequency of the bubbles; D is the average diameter of the bubbles; Po is the pressure at the bubbles; ρ is the density of the water gas mixture; μ is the Polytropic factor; and Υ is the adiabatic constant; and at least one receiver attached to the logging sonde and spaced vertically along said logging sonde from said acoustic source for detecting the arrival of the refracted P-waves.

Claim 22

The apparatus of claim 16 wherein said injector hole is located below the first location when the apparatus is positioned in the borehole.